Ferrite magnets are widely used in various applications including rotors of motors, electric generators, etc. Recently, ferrite magnets having higher magnetic properties are required particularly for the purposes of miniaturization and reduction in weight in the field of rotors for automobiles and increase in performance in the field of rotors for electric apparatuses.
High-performance sintered magnets such as Sr ferrite or Ba ferrite are conventionally produced through the following processes. First, iron oxide is mixed with a carbonate, etc. of Sr or Ba and then calcined to cause a ferritization reaction (ferrite-forming reaction). The resultant calcined clinker is coarsely pulverized, mixed with SiO2, SrCO3, CaCO3, etc. for controlling sintering behavior and Al2O3, Cr2O3, etc. for controlling iHc, and then finely pulverized to an average diameter of 0.7–1.2 μm in a solvent. A slurry containing the finely pulverized ferrite-forming material is wet-molded while being oriented in a magnetic field. The resultant green body is dried, sintered and then machined to a desired shape. To increase the properties of the ferrite magnets produced by such a method, there are the following five methods available.
The first method is a fine pulverization method. When the size of crystal grains in the sintered body is close to about 0.9 μm, a critical single magnetic domain diameter of a magnetoplumbite (M)-type Sr ferrite magnet, its iHc is maximum. Accordingly, fine pulverization may be carried out to an average diameter of 0.7 μm or less, for instance, taking into consideration the crystal grain growth at the time of sintering. This method is, however, disadvantageous in that finer pulverization leads to poorer water removal at the time of wet molding, resulting in poorer production efficiency.
The second method is to make the sizes of the crystal grains in the sintered body as uniform as possible. Ideally, the sizes of the crystal grains are made as uniformly as possible equal to the above critical single magnetic domain diameter (about 0.9 μm), because crystal grains larger than or smaller than this size have low iHc. Specific means for achieving high performance in this method is to improve a particle size distribution of fine powder. In commercial production, however, other pulverization apparatuses than ball mills, attritors, etc. cannot be used, naturally posing limitations in the level of improvement in magnetic properties by fine pulverization. Also, an attempt was recently published to produce fine ferrite powder having a uniform particle size by a chemical precipitation method. Such method is, however, not suitable for industrial mass production.
The third method is to improve crystal orientation affecting magnetic anisotropy. Specific means in this method is to improve the dispersion of ferrite particles in a fine powder slurry by adding a surfactant, or to increase the intensity of a magnetic field at the time of orientation, etc.
The fourth method is to improve the density of a sintered body. A Sr ferrite sintered body has a theoretical density of 5.15 g/cc. Sr ferrite magnets commercially available at present have densities ranging from 4.9 g/cc to 5.0 g/cc, corresponding to 95–97% of the theoretical density. Though improvement in Br is expected by increasing the density of a ferrite magnet, a higher density than the above level needs such density-increasing means as HIP, etc. However, the use of such density-increasing means leads to increase in the production cost of ferrite magnets, depriving the ferrite magnets of advantages as inexpensive magnets.
The fifth method is to improve a saturation magnetization as or a crystal magnetic anisotropy constant of a ferrite compound per se, which is a main component (main phase) of the ferrite magnet. It is likely that the improvement in the saturation magnetization σs directly leads to improvement in the residual magnetic flux density Br of the ferrite magnet. It is also likely that the improvement in the crystal magnetic anisotropy constant leads to improvement in the coercivity iHc of the ferrite magnet. Though research is being carried out on W-type ferrite having a higher saturation magnetization than that of the conventional ferrite compound having an M-type crystal structure, the W-type ferrite has not been subjected to mass production because of difficulty in the control of a sintering atmosphere.
Widely used at present among the above methods for improving the properties of ferrite magnets are the first to fourth methods, though it is difficult to drastically improve the properties of ferrite having a main phase expressed by SrO.nFe2O3 by the first to fourth methods for the reasons described below. The first reason is that the above first to fourth methods include conditions lowering productivity or steps difficult to carry out from the aspect of mass production. The second reason is that further improvement in magnetic properties, particularly Br, is extremely difficult because they are close to the theoretically highest level.
Next, as a result of investigation of a hexagonal magnetoplumbite sintered ferrite magnet described in Japanese Patent Laid-Open No. 9-115715, it has been found that higher iHc cannot easily be achieved.
It may be considered as a specific means for the above fifth method to mix ferrite expressed by AO.nFe2O3, wherein A is Sr and/or Ba, with other types of metal compounds such as metal oxides to replace part of A and Fe elements in the ferrite with other elements thereby improving the magnetic properties of the ferrite.
The magnetism of the magnetoplumbite ferrite magnet is derived from a magnetic moment of Fe ions, with a magnetic structure of a ferri-magnet in which magnetic moment is arranged partially in antiparallel by Fe ion sites. There are two methods to improve the saturation magnetization in this magnetic structure. The first method is to replace the Fe ions at sites corresponding to the antiparallel-oriented magnetic moment with another element, which has a smaller magnetic moment than Fe ions or is non-magnetic. The second method is to replace the Fe ions at sites corresponding to the parallel-oriented magnetic moment with another element having a larger magnetic moment than Fe ions.
Also, increase in a crystal magnetic anisotropy constant in the above magnetic structure can be achieved by replacing Fe ions with another element having a stronger interaction with the crystal lattice. Specifically, Fe ions are replaced with an element in which a magnetic moment derived from an orbital angular momentum remains or is large.
With the above findings in mind, research has been conducted for the purpose of replacing Fe ions with various elements by adding various metal compounds such as metal oxides. As a result, it has been found that Mn, Co and Ni are elements remarkably improving magnetic properties. However, the mere addition of the above elements would not provide ferrite magnets with fully improved magnetic properties, because the replacement of Fe ions with other elements would destroy the balance of ion valance, resulting in the generation of undesirable phases. To avoid this phenomenon, ion sites of Sr and/or Ba should be replaced with other elements for the purpose of charge compensation. For this purpose, the addition of La, Nd, Pr, Ce, etc. is effective, resulting in magnetoplumbite ferrite magnets having high Br or high Br and coercivity.
When compounds of rare earth elements such as La and compounds of M elements such as Co are added to produce high-performance ferrite magnets by the fifth method, it is usual to carry out the addition of such compounds before the calcination, namely before the ferritization reaction. Such addition method is called herein “prior-addition method.” Though the ferrite magnets formed by the prior-addition method have high Br and high iHc, the squareness ratio Hk/iHc tends to remarkably decrease as the amounts of these elements added increase, particularly when R is La and M is Co. The tendency of decrease in the squareness ratio Hk/iHc by the prior-addition method is also appreciated in the case of R═La, M=Co+Zn, or M=Co+Mn. Because the critical demagnetizing field intensity decreases by decrease in the squareness ratio Hk/iHc, the ferrite magnets are likely to lose its magnetization. How easily the ferrite magnets lose their magnetization is critical particularly when the ferrite magnets are assembled in magnetic circuits for rotors, etc. Ferrite magnets with higher squareness ratio Hk/iHc are thus desired.
Therefore, high-performance ferrite magnets satisfactory both in a coercivity iHc (or coercivity iHc and residual magnetic flux density Br) and in a squareness ratio Hk/iHc are desired.
Accordingly, an object of the present invention is to provide a high-performance ferrite magnet having substantially a magnetoplumbite crystal structure, which has higher coercivity iHc (or higher coercivity iHc and residual magnetic flux density Br) than those of conventional ferrite magnets and also has high squareness ratio Hk/iHc, thus useful in wide varieties of magnet applications such as rotors for automobiles and electric appliances, magnet rolls for photocopiers, etc., and a method for producing such a high-performance ferrite magnet.
Another object of the present invention is to provide a high-performance ferrite magnet having substantially a magnetoplumbite crystal structure, which has higher coercivity iHc (or higher coercivity iHc and residual magnetic flux density Br) and higher squareness ratio Hk/iHc than those of conventional ferrite magnets, and also has a microstructure in which the concentration of an R element is high in crystal grain boundaries, and a method for producing such a high-performance ferrite magnet.